Re: Screening for contaminated dosimeters

[Bob Flood <bflood@SLAC.Stanford.EDU>]

At 01:32 PM 11/20/97 -0600, you wrote:
>My question is: Do other institutions check their badges routinely for
>contamination? If so, or if not, why? What else could have caused
>abnormal counts on maybe 10 percent of the badges?

Both accrediting systems, NVLAP and DOELAP, require screening for
contamination before processing dosimeters. A commercial lab with NVLAP
accreditation is the organization responsible for doing the survey, since
it's their accreditation they are trying to maintain. Most labs (commercial
and private) don't keep records of the surveys unless something is detected.

If a shipment to the lab contained only one control dosimeter, you have a
couple of possibilities, both of which are caused by the use on only one
control badge.

1) As someone else has already mentioned, the shipment may have been near
radioactive material while in transit. If the control badge was on the side
farther from the source, it will underestimate the correction needed for
the dosimeters nearest the source, which could result in a fraction of the
shipment showing positive doses. Use of more controls distributed among the
field badges would minimize this possibility.

2) This may be a background subtraction problem. If the number of
background dosimeters is small, variations in background measurements will
inevitably cause false positives and false negatives in the reported data.
The vendor may use an average background measured in their lab, they may
use data from your site, they may use a combination of these, etc. To judge
whether the background subtraction method may be contributing to this, you
need to understand how your supplier does this.


Bob Flood
Stanford Linear Accelerator Center
(415) 926-3793     bflood@slac.stanford.edu
Unless otherwise noted, all opinions are mine alone.